German Offenlegungsschrift 1,729,345 relates to a method in which the joint between such tubes is produced by welding the two tubes from the outside in the vicinity of the adjacent abutting surfaces (secant surfaces). In this method, it is impossible to avoid formation of a crack in the internal saddle area of the abutting tube sections due to the thermal expansion and stress developing during welding, so that during use of the extruder so made, material to be processed, especially thermoplastic, can enter such a crack, such thermoplastic material decomposing with time and then contaminating the material to be processed.
In addition, the presence of such a crack leads to premature wear of the edges in the area of the saddle. To eliminate these problems, German Offenlegungsschrift 1,729,345 also teaches the lengthwise division of the housing of the screw conveyor, insertion of tube segments serving as liners, and compressing them with tensioning screws joining the parts of the housing. However, this is an unsatisfactory solution. Adequate tensioning results in elastic and (under certain conditions) even residual deformations in the vicinity of the saddle points, i.e., especially in the wearing coating which serves for protection. In addition, cracks may result in the area of the abutting surfaces, into which the softened plastic can penetrate.
German Offenlegungsschrift 2,231,046 teaches the use of a wearing insert made of two tubes welded together along their milled abutting surfaces (secant surfaces), wherein the tubes are provided with recesses on the outside along the butt seams, the recesses alternating with smooth stamped surfaces, wherein the welded seams joining the tubes are provided only in the area of the recesses. By this means, it is intended that the abutting surfaces be so close together that none of the material being processed in the screw conveyor can penetrate between the abutting surfaces. However, here again it is unavoidable that aggressive gases or condensates from the molten plastic can enter the crack between the abutting surfaces and corrode the surfaces. In addition, this method of manufacture is extremely costly.
U.S. Pat. No. 3,010,151 proposes the utilization of two tubes cut along the secants and joined with the secant surfaces (abutting surfaces) to form relatively pointed saddles, the tubes being provided on the inside with a wearing coating and also provided on the outside with a through recess in the area of the abutting surfaces, with the tubes being welded together in this recess. A thin layer of inert metal is soldered onto the welded seam and the remainder of the recess is then potted with lead. The solder layer of inert metal (which can be silver, gold, tin, copper, or the like) serves to prevent the aggressive components of the material being processed in the screw conveyor from penetrating through the crack between the abutting surfaces, and then reaching the housing of the screw conveyor and causing corrosion therein. However, this procedure is difficult and costly, requiring a great expenditure of work; furthermore, even this complex procedure has not been sufficiently successful since aggressive materials from within still have a tendency to penetrate between the adjoining parts causing corrosion and ultimately destruction, as well as contamination of the material being processed.